As the demand for high performance integrated circuit devices continues to increase, designers have reduced circuit geometries in order to obtain improved performance. As the gate lengths of MOS transistors shrink to sub-half-micron dimensions, the switching speed of the transistors increases dramatically. To take full advantage of the increased speed of the transistors, electrical interconnect structures having high electrical conductivity must also be provided. The performance of advanced integrated circuit devices is often limited by the electrical conductivity of metal interconnects, which electrically couple the various device components of an integrated circuit.
In the past, aluminum, and its alloys, have been widely used for metal interconnect structures. However, as device geometries continue to be reduced, the number of devices which must be electrically interconnected has increased. The increased number of electrical interconnections required for advanced integrated circuit designs necessitates the formation of extremely narrow interconnect leads. The utilization of aluminum and its alloys for high density interconnect formation is limited by the tendency of aluminum to undergo thermally induced voiding and electromigration. An additional problem associated with aluminum metallurgy is the relatively higher electrical resistance of aluminum alloys as compared to other electrically conductive metals.
To overcome the limitations associated with the use of aluminum for electrical interconnects, other metals, such as copper, gold, and silver have been proposed as a substitute for aluminum and its alloys. Copper offers a desirable alternative to aluminum, because of its low resistivity and resistance to electromigration. However, copper diffuses readily in materials commonly used in integrated circuit fabrication, such as silicon and silicon dioxide. Additionally, copper does not adhere well to many other metals nor to insulators such as silicon dioxide and silicon nitride. The aforementioned characteristics of copper prevent the relatively straight forward formation of copper leads in a manner analogous to that used in the formation of aluminum interconnects. Therefore, the implementation of copper for the formation of electrical interconnects requires that special processes and materials be provided to overcome the problems of diffusion and adhesion associated with the use of copper.